Various different tomographic techniques for imaging bodies for medical and other purposes are known and have found practical application. These include X-ray computed tomography (CT), magnetic resonance imaging (MRI), single photon emission computed tomography (SPECT), positron emission tomography (PET), and electrical impedance tomography (EIT).
Considering the last-mentioned technique in more detail, EIT involves the application of electrical signals to peripheral points of a body under investigation and the measuring, at other peripheral points, of electrical voltages or currents. The measurements provide data sets which can be processed to form an image representing the distribution of electrical conductivity or resistivity over a section of the body. EIT provides a relatively inexpensive tomographic technique in terms of the equipment required and does not involve any of the risks associated with some other techniques, such as radiography. A description of an EIT tomographic imaging system can be found in U.S. Pat. No. 4,617,939, where the method of `backprojection` is disclosed, used for reconstructing the image from the data sets. This patent also teaches the method as applied to the study of rapid changes in the internal state of the body, enabling dynamic imaging of, for example, respiration response, cardiac response or gastric functioning.
Technical advances in hardware and software since the date of invention of the technique described in U.S. Pat. No. 4,617,939 now enable the production of EIT images in real time using parallel processing. Data acquisition at 25 frames per second (fps) is thus possible, enabling a clinician to display on a video monitor a real time colour image sequence representing the temporal changes of impedance within the body.
To be useful, the displayed image must appear large enough on the screen to be viewable from a reasonable distance. As presently used, a 16 electrode EIT system provides only 104 independent measurements at any one time. The backprojection image reconstruction matrix used generates 208 image pixels from these measurements, as this number of image pixels can conveniently be arranged within a circle inscribed within a square array of 16.times.16 pixels. If this image were displayed directly on a typical image display of, say, 512.times.512 display pixels, with each image pixel occupying one display pixel and the 208 occupied pixels being located immediately adjacent one another, the displayed image would be too small for convenient viewing. On a typical 14-inch diagonal CRT monitor the image would be only 7 mm in diameter.
To produce a larger and hence more useful displayed image, the relatively small number of image pixels must be spaced out over the array of many display pixels, and some means of image interpolation is required for allocating image display values to the display pixels located between the image pixels. An interpolation scheme should provide an acceptable transition between neighbouring image pixels whilst performing the required computation sufficiently quickly to enable the results to be displayed in real time, without requiring powerful and expensive hardware.
Two previous interpolation schemes used in EIT image display include nearest-neighbour interpolation and bilinear interpolation. In the former, a display pixel is assigned the image value of its most nearly adjacent image pixel, thus effectively merely enlarging each image pixel on the display. This method, also called `zero-order interpolation`, involves minimal computation time as a simple pattern remains constant in time, but can have the drawback of producing undesirable artifacts, such as very sharp boundaries within the displayed image which are highly distracting to the eye.
The method of bilinear interpolation involves calculating the image value of each display pixel for each frame as a linearly weighted sum of the values of the several most nearly adjacent image pixels, for example of the four nearest image pixels if these are distributed on a square lattice. Boundaries between image pixels are much less obtrusive than those obtained using nearest-neighbour interpolation. However, for each frame, a new image value has to be calculated for each display pixel, and this involves considerable calculation time. The image value of a particular display pixel is of course not necessarily that of any of the neighbouring image pixels, the values for the interpolated display pixels being in general both different from and more numerous than those of the image pixels. With the equipment currently commonly available it is unlikely that in the 40 ms frame time an image large enough for viewing by more than one or two persons is feasible. On the 14-inch CRT monitor the largest image producible in the available time per frame is approximately 50 mm in diameter.